Ryuuichi Sudou is a college student nearing his final year. He receives free tickets for a mystery tour from his friend Takashi Seiguu and decides to participate during summer vacation. Though typically skeptical of such offers, Ryuuichi feels an inexplicable pull to join this trip, described as sensing "as if something was calling me." His childhood friend Wakaba insists on accompanying them despite his initial reluctance.

Personality-wise, Ryuuichi displays some perverse tendencies but is fundamentally a "nice guy." He demonstrates concern for others and engages in methodical exploration of Oka Island alongside fellow tour members. His background includes unspecified "tragic memories" linked to the island, a shared experience among several other participants.

On Oka Island, Ryuuichi witnesses and experiences extreme situations, including bondage, torture, and murder perpetrated by the island's owner, Mr. Okouchi. He participates in efforts to resolve the crisis, culminating in a supernatural phenomenon that provides closure to the traumatic events.